We have found that the concentration of the prostaglandin-forming cyclooxygenase in the uterine caruncles of cycling ewes fluctuates in parallel with the rate of secretion of the uterine luteolysin, PGF2 alpha. These results along with those of others suggest a model which predicts that the uterine cyclooxygenase increases in a specific luteolysin-forming cell just prior to luteolysis in response to sequential exposure of this cell to progesterone and estradiol; the net effect of increasing the cyclooxygenase, which catalyzes the rate-determining step in prostaglandin formation, is to increase the rate of conversion of available substrate acid to prostaglandin endoperoxides which are then rapidly converted to PGF2 alpha. We plan three additional experiments to test this hypothesis. First, we plan to isolate populations of endothelial cells and stromal cells from uterine caruncles and determine in which of these populations the cyclooxygenase fluctuates during the estrous cycle. We will then determine if the implicated cell type actually produces the luteolysin, PGF2 alpha, or instead preferentially forms other prostaglandin derivatives. In the other two experiments we will evaluate the effect on the uterine cyclooxygenase of: (a) inducing premature luteolysis in cycling progesterone-primed ewes by treatments with estradiol and (b) blocking luteolysis by preventing normal estradiol secretion using X-irradiative destruction of the ovarian follicles and compare any changes with those that normally occur on days 13-15 in cycling ewes. Other experiments are proposed to test for the presence of plasma factors which will cause the selective release of arachidonic acid from its esterified form in isolated luteolysin-forming cells whose lipids have been prelabeled with both H3-arachidonic and C14-linoleic acids. Our experiments are designed to determine how the production of the uterine luteolysin, PGF2 alpha is regulated.